This proposal describes the synthesis and validation of the first example of a biochemical- responsive ultrasound contrast agent, a potentially groundbreaking advance for deep-tissue imaging of intravascular disease. By improving the power of ultrasound, completion of this multidisciplinary proposal would add value and power to an inexpensive, widespread technique and thus has excellent potential for clinical translation. In addition, this mechanism of activation may be translated to other applications like direct enhancement of sonolytic therapy or site- specific release of therapeutic agents. In this proposal, This proposal describes the synthesis and validation of the first example of a biochemical- responsive ultrasound contrast agent, a potentially groundbreaking advance for deep-tissue imaging of intravascular disease. concern for this country. Acute thrombi may form quickly and develop into worse conditions, potentially leading to patient death if left untreated. Detection is preferably performed using compression ultrasound, a painless, non-invasive outpatient procedure that has good predictive value for symptomatic, proximal vein thrombosis. However, ultrasound cannot differentiate between older clots that should not be treated and acute thrombi that must be treated immediately. The team of PIs has developed a new type of microbubble that remains invisible to ultrasound under normal conditions but becomes detectable only after exposure to certain chemical stimuli. Prior to activation, the microbubble is coated by a rigid, crosslinked polymer shell that dampens the microbubble's response to ultrasound. Treatment with a specific stimulus removes the crosslinks, leaving a flexible, freely oscillating microbubble. The resultant microbubble can be detected via its harmonic oscillations with excellent specificity. Microbubbles will be designed to exhibit sensitivity to thrombin, a protease involved in the coagulation cascade. The microbubbles will be coated with polymer shells crosslinked by DNA aptmaters sensitive to thrombin. Next, their behavior in ultrasound will be examined in real-time on the single bubble level through special equipment previously designed by the PI to create ultrasound pulse programs for optimized signal-to-noise imaging. These thrombin-sensitive ultrasound contrast agents will then be validated in an ex vivo flow model and an in vivo rabbit thrombosis model to detect actual acute thrombi. Thus, a new type of microbubble will be tuned to a medically-relevant system and validated in vivo with excellent chance for translation.